Sirius appears as a tall and thin woman. Her hairstyle is about the same as Altair's. Her irises are blue and her outfit is slightly different from Altair's. Her weapon, unlike Altair's arsenal of swords, is a single, gigantic, two-sided spear.
She was made by the Creators as a weapon against Altair. Her character was foreshadowed in a few episodes created by the Creators before the events of the Border World Colliseum. But as a result from her character not having much of a story, she did not have a personality and rarely spoke, unlike Altair. She was summoned by Meteora and Rui Kanoya shortly after Selesia's death. Sirius immediately fired a shot at Altair after being summoned into the colliseum, which was powerful enough to knock Altair back. She then moves towards Altair quickly and throws her spear at Altair, which Altair struggled to deflect. When Altair realises that Sirius was one of the designs that her creator had made, she becomes enraged and uses one of her Holopsicon skills in an effort to defeat Sirius, but her plan backfired as Sirius launched the very same attack which obliterated Altair momentarily.
Altair resurrects shortly after and dashes towards Sirius in rage, only to be frozen by one of Sirius' spells. Altair tries to get free as she learns of Sirius' next move. Once Sirius has gotten to Altair, she stabs herself in the gut while casting one of her holopsicon spells, in an attempt to erase Altair's existence, leaving Sirius instead, and therefore defeating Altair. But the plan backfires as Sirius had not received the audience's Acceptance and her Altair comes into her body instead. Altair, now in Sirius' body, regains consciousness and walks toward her hat. Soon, she picks it up and wears it, then she peirces Yuya Mirokuji in the back with some of her swords.